ALBANY — A disability-pension sweetener for NYPD cops vetoed five years ago by then-Gov. David Paterson was quietly reintroduced last month and could slip through this year, a top fiscal watchdog warned Monday.

The Citizens Budget Commission was so worried that the bill would be passed in the final days of the legislative session next week — costing city taxpayers about $35 million a year — that its president fired off a letter to Mayor de Blasio, asking him to “reaffirm” his “commitment to fiscal responsibility” by opposing it.

In her letter, CBC chief Carol Kellermann pointed out that the pension changes enacted during the previous City Hall administration are supposed to save the city an estimated $31 billion over 30 years.

The mayor said he couldn’t comment because he hadn’t seen the measure — even though it was introduced May 12.

The bill’s state Senate sponsor, Marty Golden (R-Brooklyn), also said he wasn’t ready to discuss it in detail.

“We are looking at it and trying to get it out,” said Golden, who is a former cop. “We’re discussing it, and I don’t want to go any further than that as to the purpose of the changes.”

Peter Abbate (D-Brooklyn), the Assembly sponsor, said he signed on as a matter of fairness.

“My understanding is the cost can be expensive,” he said. “But we really have to watch and make sure those that are really injured get the disability.”

Abbate dismissed the huge hit on the city treasury, saying, “This is the excuse that is always given — ‘It costs too much, people get it unfairly’ — and then the good suffer.”

The bill would reverse pension savings made during the Bloomberg administration, when the retirement date for cops hired after July 1, 2009, was extended from 20 to 22 years of service, and disability payments were reduced from 75 percent of final pay to 33¹/₃ to 44 percent, with an additional offset for Social Security.

Kellermann warned that reversing course would strike a severe blow against reform.

“We fear it could be the beginning of the unraveling of the pension reform that Gov. Paterson and Gov. Cuomo enacted,” she said.

She added that more than one-third of police retirees — about 15,000 individuals — are now collecting disability pensions.

This year’s legislative session is scheduled to end June 19 — at which point many bills are expected to be rushed through with little scrutiny.